In the field of wireless communication, there is a growing demand for a broadband wireless access system that enables efficient, high-speed, high-volume communications. For downlink in such a system, a multicarrier scheme such as orthogonal frequency division multiplexing (OFDM) is expected to be used to achieve high-speed, high-volume communications while effectively restraining multipath fading. Also, in next generation systems, use of frequency scheduling is proposed to improve the frequency efficiency and thereby to increase the throughput.
As shown in FIG. 1, in next generation systems, a system frequency band is divided into multiple resource blocks (in this example, three resource blocks) each including one or more subcarriers. The resource blocks may also be called frequency chunks. Each terminal is allocated one or more resource blocks. In a frequency scheduling method, to improve the transmission efficiency or the throughput of the entire system, resource blocks are allocated preferentially to terminals with good channel conditions according to received signal quality or channel quality indicators (CQIs) measured based on downlink pilot channels and reported by the terminals for the respective resource blocks. A pilot channel is a signal known to both the sending end and the receiving end, and may also be called a reference signal, a known signal, and a training signal. When frequency scheduling is employed, it is necessary to provide the terminals with scheduling information indicating the results of scheduling. The scheduling information is reported to the terminals via control channels. A control channel may also be called an L1/L2 control signaling channel, an associated control channel, or a physical downlink control channel (PDCCH). The control channel is also used to report a modulation scheme (e.g., QPSK, 16 QAM, or 64 QAM) and channel coding information (e.g., channel coding rate) used for the scheduled resource blocks as well as information regarding hybrid automatic repeat request (HARQ). For the structure of control channels used in such a mobile communication system, see, for example, 3GPP, TR25.848, “Physical layer aspects of UTRA High Speed Downlink Packet Access” and 3GPP, TR25.896, “Feasibility study of enhanced uplink for UTRA FDD”.
Here, when a resource block common to all terminals is statically allocated for a control channel, some terminals cannot receive the control channel with good quality because channel conditions of a resource block differ from terminal to terminal. Meanwhile, distributing a control channel to all resource blocks may make it possible for all terminals to receive the control channel with certain reception quality. However, with this method, it is difficult to further improve the reception quality. For these reasons, there is a demand for a method of transmitting a control channel with higher quality to terminals.
In a system where adaptive modulation and coding (AMC) is employed, i.e., where the modulation scheme and the channel coding rate are adaptively changed, the number of symbols used to transmit a control channel varies from terminal to terminal. This is because the amount of information transmitted per symbol varies depending on the combination of the modulation scheme and the channel coding rate. For a next generation system, it is also being discussed to send and receive different signals by multiple antennas provided at the sending and receiving ends. In this case, control information such as scheduling information as described above may be necessary for each of the signals transmitted by the multiple antennas. In other words, in such a system, the number of symbols necessary to transmit a control channel may vary from terminal to terminal and also vary depending on the number of antennas used by the terminal. When the amount of information to be transmitted via a control channel varies from terminal to terminal, it is necessary to use a variable format that can flexibly accommodate various amounts of control information to improve resource use efficiency. However, using a variable format may increase the signal processing workload at the sending and receiving ends. Meanwhile, when a fixed format is used, it is necessary to provide a dedicated control channel field that can accommodate the maximum amount of control information. In this case, even if a control channel occupies only a part of the control channel field, the resources for the remaining part of the control channel field cannot be used for data transmission and as a result, the resource use efficiency is reduced. For these reasons, there is a demand for a method to transmit a control channel in a simple and highly efficient manner.
However, related-art methods of transmitting a control channel still cannot meet the above demands.